Fiber optic sensors have been successfully used in many applications such as structural monitoring, acoustic sensing, temperature and pressure sensing. Due to their immunity to electromagnetic interference, wide temperature range, and capability for long range interrogation, fiber optic sensors offer numerous advantages as compared to other sensing technologies. Fiber optic sensors are particularly well-suited for applications such as down-hole drilling, where high pressure, high temperatures, and extreme chemical environments are prevalent.
Based on fiber optics, several types of point sensors have been reported among which can be highlighted: fiber Bragg grating sensors, multimode interference sensors, and twin-core fiber sensors, as a few examples. A variety of multimode interference devices have been developed for fiber optic sensing; specifically, single mode-multimode-single mode devices, wherein a multimode fiber is spliced between two single mode fibers. These devices have shown great promise in high temperature and various other sensing applications. However, these devices offer little control over the interference produced, and/or require complicated fiber geometries using suspended cores or photonic crystal fiber, making their industrial implementation unpractical.
Zhao et al., All-solid multi-core fiber-based multipath Mach-Zehnder interferometer for temperature sensing, Appl. Phys. B (2013) 112:491-497 have reported an in-fiber integrated multipath Mach-Zehnder interferometer (m-MZI) fabricated by fusion splicing a segment of all-solid multi-core fiber (MCF) between two sections of single mode fiber-28 with a well-controlled lateral offset at the splice points for temperature sensing applications. Characteristics of the disclosed apparatus and methods include a large pitch—distance between each core, resulting in little or no optical interaction between cores, core index differences, off-center splicing wherein light launched into the cladding excites propagation modes in the central core, ambient cores, and cladding of the MCF, thus leading to multi-path interference between these modes, high insertion loss, a relatively long length of multicore fiber, and small modulation depths.
P. Rugeland and W. Margulis, Appl. Opt. 51, 6227 (2012) reported twin-core fiber devices to accurately measure elevated temperatures up to 700° C. However, many applications require accurate measurement of temperatures greater than this value.
A solution to the aforementioned problems, in a form that is inexpensive, durable, accurate, sensitive, providing high resolution, reproducible measurements, compact, stable, and reliable would be beneficial and advantageous, particularly by improving the performance of multimode interference optical fiber sensors, enabled by the novel multicore fiber (MCF) devices, methods, and applications disclosed as embodiments herein.